The present invention relates to urethane elastomers and methods for their preparation.
Polyurethane is a versatile class of polymers with a variety of applications because its properties can be tailored simply by varying the components from which it is constructed (the rigid diols, the flexible polyols, and the polyisocyanate). Ulrich, H., "Urethane Polymer", in Ency. Chem. Tech., Vol. 23, pp. 576-606 (Wiley Intersciences) (1983). Polyurethanes are used as adhesives, coatings, elastomers, foams, and fibers.
Polyurethane elastomers can be either thermoplastic or thermosetting. All linear segmented urethane elastomers are thermoplastic with good tear strength, impact resistance and other excellent mechanical properties. Finelli, A., et al., "Thermoplastic Elastomer", in Ency. Chem. Tech., Vol. 8, pp. 626 et seq. (Wiley Intersciences) (1979). These properties are due in part to the strong hydrogen bonding between the hard segments of neighboring chains. Normally the thermoplastic polyurethanes are prepared by first forming the isocyanate-capped prepolymer of a flexible polyetherdiol or polyesterdiol, followed by adding the chain extender, which is the short chain diol, to form the high molecular weight polyurethane. Alternatively, the thermoplastic elastomers can be constructed by mixing the ingredients all at once, but with somewhat less useful physical properties.
In the two-step process to form the urethane elastomer, the length of the hard segment is relatively uniform in size. But when it becomes necessary to construct the hard segment before forming the soft segment, it is important to control the mode of reaction to preserve the excellent properties of polyurethane such as tear strength and toughness. A more uniform hard segment size enhances the attractive forces, i.e., hydrogen bonding, resulting in improved properties.
One method to control the size of the hard segment is by using an aromatic diisocyanate with differential reactivity between the two isocyanate groups. A prominent example of such a compound is tolylene diisocyanate ("TDI"). This is possible because of electronic and steric effects. However, this approach is not possible when using aliphatic diisocyanates or aromatic diisocyanates wherein the isocyanate groups are attached to different aromatic rings.